The present invention relates to the field of combustion of kraft black liquor and to chemical recovery in kraft recovery boilers.
The kraft black liquor recovery boiler is a critical component in the production of paper pulp. Two functions are associated to the recovery boiler: the combustion of the organic materials contained in the black liquor for the production of heat and steam, and the conversion of the inorganic chemicals of the black liquor into a smelt which consists mainly of sodium carbonate (Na2CO3), sodium sulfide (Na2S), and a small amount of sodium sulfate (Na2SO4). In further steps of the pulping process, the smelt is converted into cooking liquor, the chemical used to transform wood chips into pulp. This transformation in turn produces black liquor that must be disposed of and recycled in the recovery boiler. One of the most important functions of the recovery boiler is to convert the sodium and sulfur content of the black liquor into sodium sulfide. The efficiency of this conversion is expressed as the reduction efficiency, defined as the ratio of sodium sulfide (Na2S) in the smelt to the total of sodium sulfide and sodium sulfate (Na2SO4) in the smelt. Operation with the highest reduction efficiency is desirable.
Because recovery of the chemicals contained in the black liquor is so important in the pulping process, the recovery boiler is often the bottleneck in increasing the pulping capacity of a mill. Insufficient capacity to burn and recover chemicals from black liquor yields a deficiency in cooking liquor, that can force mills to slow down production, and/or ship black liquor to other mills that have excess recovery capacity, and/or to buy make up chemicals to compensate for the lack of cooking liquor. All of these activities are detrimental to the cost of the pulp produced in the mill.
Adding additional recovery boiler capacity can be obtained by installing a new recovery boiler, or another means of burning black liquor, such as a gasifier, or a fluidized bed. These solutions are avoided when possible, because they are associated with high capital costs, and represent an additional unit to operate and maintain. Preferred solutions are solutions that retrofit an existing recovery boiler and add black liquor combustion capacity without excessive capital costs or time required to install.
In recovery boilers, black liquor combustion occurs by in-flight burning, and char bed burning. During in-flight burning, the water from the black liquor droplets is evaporatedxe2x80x94drying stagexe2x80x94, organic materials are volatilized and burnxe2x80x94volatilization stagexe2x80x94, and the solid residue known as char, burnsxe2x80x94char burning stagexe2x80x94. The unburned residue falls onto the char bed located at the bottom of the furnace, where combustion and chemical recovery are completed. To sustain combustion, air is injected at various heights in the furnace: primary air is the lowest level of air injection, situated at the lower level of the char bed; some boilers have a secondary air level is positioned above the top of the char bed. Both the primary and the secondary air levels are located below the level of the liquor guns used to spray coarsely atomized black liquor into the furnace. Modern recovery boilers have a third level of air injection which is located above the level of the liquor guns (tertiary air). Some very large boilers have a fourth air injection level (quaternary air) above the liquor gun elevation.
One important limitation to the increase of throughput of a recovery boiler is fouling of the convection heating surfaces. When attempting to increase the black liquor feedrate, the quantity of combustion air required must be increased, which results in an increased volume of combustion products, increased vertical gas velocity, and additional entrainment of liquor, char, and/or smelt particles (carryover) would occur. This problem is worsened when the combustion chamber height is relatively small. Fouling of the upper sections of the boiler by the carryover material can result in the plugging of the flue gas passages, and will eventually cause a boiler shut down. Boiler shut downs and start-ups are complicated operations that should be avoided as much as possible. In addition, during boiler shut down, a temporary solution for black liquor disposal and cooking chemical make up must be found, which is often a cost penalty.
Another difficulty of increasing boiler throughput is the need to supply additional combustion air and handle more flue gas. Fan capacity limitations can be found, either on the combustion air supply side, the exhaust side, or both, that will prevent combustion of additional black liquor in an existing furnace.
Solutions to the previous limitations can be found by minimizing the excess air (excess air is the volume of air over and above the volume of air needed to complete combustion) for combustion. However, lowering the excess air may result in increased pollutant emissions, such as carbon monoxide (CO) and sulfur compounds (TRS). In older boilers with two levels of air, where the tangential injection of combustion air does not provide good penetration of the air into the center of the furnace, adding a third level is a convenient solution to reduce the excess combustion air because three level air injection gives a more efficient distribution and mixing of the air in the different areas of the boiler. Two level air furnaces that are overloaded have also shown not to be as efficient at completely oxidizing CO and sulfur compounds and thus yield higher pollutant emissions. For this reason, and the obligation of operating with a large excess air to compensate for the poor mixing of the combustion air with the combustibles, the boilers with two levels of air are progressively retrofitted by more efficient three level air systems. On three level air systems, retrofitting the boiler with high velocity air nozzles such as described in US Pat. No. 4,940,004 in the secondary and tertiary levels of the furnace can improve mixing and allow operation with even less excess air.
Reducing the excess air has also a positive effect on the thermal efficiency of the boiler, defined as the ratio of energy in the steam produced in the boiler over the amount of energy in the black liquor.
Oxygen injection has been proposed to further reduce the air and the flue gas volumes in recovery boilers. In a boiler operated with an oxidant which oxygen concentration exceeds that of the air, more black liquor can be burned with a constant volume of air and flue gas. For example, in a review article entitled xe2x80x9cIncreasing Recovery Boiler Throughputxe2x80x9d by T. M. Grace, published in the November 1984 issue of Tappi Journal, given as a reference, the author cites oxygen enrichment as a means of reducing the volume of air and flue gas for a given heat release. In U.S. Pat. No. 4,823,710, an oxygen injection method is described where combustion is improved by introducing an oxygen containing gas, preferably with an oxygen concentration higher than air, from at least one location remote from the boiler sidewalls. U.S. Pat. No. 4,857,282 discloses a method to use oxygen enrichment in the primary and secondary air system of the boiler. However, the ""282 patent does not disclose or suggest oxygen enriched air injection at the secondary or tertiary air levels without injection at the primary air level.
In accordance with the present invention, methods are presented to increase the throughput of recovery boilers equipped with at least two levels of injection of air without increasing the carryover of inorganic materials in the recovery boiler in order to prevent plugging of the convective sections of the boiler. Another advantage of the methods of the invention is lowering the emissions of gaseous pollutants from the recovery boiler. Another advantage of the methods of the invention is to improve the furnace control and stability of operation, and to eliminate or reduce the need for an auxiliary fuel to sustain the combustion of low heat content black liquors, and the chemical reduction efficiency in the furnace is increased.
One aspect of the invention is a method to increase the throughput of a recovery boiler equipped with at least two levels of injection of air, the method comprising improving the thermal efficiency of the boiler with oxygen enrichment of the air in at least one level of the combustion air system, at or above the secondary air level.
A second aspect of the invention is a method to retrofit black liquor recovery boilers having a two level air injection system with a third level of oxidant injection below or at the same level as the original secondary air, and oxygen enrichment applied to at least the original secondary air stream and said third level. A preferred method is for the third level to be placed at the same level as black liquor injector ports. Another preferred method comprises injecting the third level of oxygen enrich air at the same level as the secondary air injection ports. A more preferred embodiment of this method is to place the third level at a level lower than the level of the black liquor injection guns and higher than the primary air level. Once retrofitted to three levels of air injection, the two upper levels of air injection are re-named: said third level becomes the secondary level, and said original secondary level becomes the tertiary level.
In a third method of the invention applicable to boilers with at least three air injection levels, or boilers with two air injection levels retrofitted to three levels as described above for the third aspect of the invention, oxygen enrichment is applied to at least the secondary and the tertiary air levels. In preferred methods of the invention, oxygen enrichment is applied to the primary air level in addition to the secondary and tertiary air levels.
In a fourth method of the invention applicable to boilers with at least four air injection levels, oxygen enrichment is applied to at least the secondary and one or more of the two upper air levels. In preferred methods of the invention, oxygen enrichment is applied to the primary air level in addition to the secondary and fourth air levels.
In preferred aspects of the invention, oxygen enriched air is injected at a velocity greater than 100 feet per second (ft/s) where oxygen enrichment is applied. More preferably, the oxygen enriched air stream is injected at a velocity greater that 200 ft/s in the secondary oxidant stream, and at a velocity greater than 200 ft/s in the tertiary oxidant stream.
In preferred aspects of the invention that apply to boilers with four levels of combustion air, oxygen enriched air is injected at a velocity greater than 100 ft/s where oxygen enrichment is applied. More preferably, the oxygen enriched air stream is injected at a velocity greater that 200 ft/s in the secondary oxidant stream, and at a velocity greater than 250 ft/s in the fourth oxidant stream.
A fifth aspect of the invention is a method of controlling the oxygen concentration in the flue gas of a recovery boiler when oxygen enrichment of the combustion air is applied, the method being applicable to boilers with at least three levels of air injection, or a recovery boiler with an original two level air injection system retrofitted to three levels as described above, said method including the steps of:
a) supplying oxygen flows to at least two combustion air levels of the recovery boiler, said two combustion air levels being different from the primary air level, for oxygen enrichment of the said two combustion air levels;
b) selecting a desired oxygen concentration in the flue gas called set point concentration,
c) sensing the oxygen concentration in the flue gas;
d) adjusting the oxygen flow injected in the tertiary combustion air level, in order to maintain the sensed oxygen concentration in the flue gas at about the set point oxygen concentration, while maintaining the flow of oxygen in the secondary level combustion air constant.
Secondary and tertiary air levels are as defined in the above.
In a sixth aspect of the invention, a method is provided for controlling the oxygen concentration in the flue gas of a recovery boiler when oxygen enrichment of the combustion air is applied, the method being applicable to boilers with at least four levels of air injection. This seventh method comprises the steps of:
a) supplying oxygen flows to at least two combustion air levels of the recovery boiler, said two combustion air levels being different from the primary air level, for oxygen enrichment of the said two combustion air levels;
b) selecting a desired oxygen concentration in the combustion products called set point concentration;
c) sensing the oxygen concentration in the flue gas;
d) adjusting the oxygen flow injected in the upper most combustion air level, in order to maintain the sensed oxygen concentration in the flue gas at about the set point oxygen concentration, while maintaining the flow of oxygen in the other level of combustion air constant.
A seventh aspect of the present invention is a method to improve the combustion stability or chemical recovery of a recovery boiler where oxygen enrichment is applied to at least one level of the combustion air system at or above the secondary air, comprising the steps of:
a) supplying oxygen flows to the primary combustion air level of the recovery boiler for oxygen enrichment of the primary air,
b) sensing either one or all of the following quantities: reduction efficiency of the smelt, sulfur dioxide SO2 concentration in flue gas, or bed temperature;
c) adjusting the oxygen flow injected in the primary combustion air level, in order to obtain at least one of the following effects on either or all of the following quantities: reduction efficiency above 90% and minimize SO2 emissions.
An eighth aspect of the present invention is a method to improve the combustion stability or chemical recovery of a recovery boiler where oxygen enrichment is applied to at least one level of the combustion air system at or above the secondary air level, the method comprising the steps of:
a) sensing either one or all of the following quantities: the reduction efficiency of the smelt, the sulfur dioxide SO2 concentration in the flue gas, or the bed temperature b) adjusting the oxygen flow injected in the secondary combustion air level, in order to obtain the following effects on either or all of the following quantities: keep the reduction efficiency above 90%, minimize the SO2 emissions.
In the methods of the invention, when oxygen enrichment is applied to two or more levels of combustion air, the oxygen concentration in the oxidant in each level of oxygen enriched air injection can be controlled independently.
A ninth aspect of the invention is a method of controlling temperature profile in a recovery boiler when oxygen enrichment of the combustion air is applied, said method including the steps of:
a) supplying oxygen flows to at least two combustion air levels of the recovery boiler, said two combustion air levels being different from the primary air level, for oxygen enrichment of the said two combustion air levels
b) selecting an optimal temperature profile for the boiler based on the prior knowledge of the boiler operation, called set point temperature profile,
c) sensing average temperatures at different levels of the boiler with an optical technique, and inferring a temperature profile for the boiler,
d) adjusting the oxygen flow injected in said at least two combustion air levels so that the measured temperature profile matches the boiler set point temperature profile.
Preferred optical techniques for the temperature measurement are based on the absorption and/or emission of sodium bearing species, such as, but not limited to: elemental sodium atom Na, sodium sulfate (Na2SO4), and sodium sulfide (Na2S). Preferred oxygen concentrations enrichment in the at least one level of the combustion air system located at or above the secondary air level are less than 30%. More preferably, the oxygen concentrations in the at least one level of the combustion air system located at or above the secondary air level is comprised between 22% and 26%.